In the past, it was the general practice for a telephone utility company to own both the signal trunk lines leading to the customer's location and the telephone instrumentation installed at the customer's site. As a consequence, the utility company was ordinarily responsible for repairing system failures wherever they occurred. The modern trend is for the customer to own his telephone instruments and to have the concomitant responsibility for their proper functioning. Accordingly, it has become desirable to have means available for remote discrimination between failures which have occurred in the company-responsible portion of the system and those that have developed in the customer-responsible portion.
Heretofore, devices have been proposed which, when installed in the signal input to the customer location, afforded the capability of selectively and remotely disconnecting a given customer's station from the system trunk lines whereby to permit the accomplishment of a test procedure. These prior art station-isolation devices, however, have commonly incorporated mechanical relays and have uniformly operated on comparatively high, direct current potentials, on the order of 130 volts. These prior art devices have proved to be short-lived, unreliable and subject to spurious activation, as for example by the 130 volt direct current potential of such telecommunication services as coin-operated telephones.